Is it possible to restrict my editing just to one channel of a layer (I'd like to show just R channel of selected layers) - this is basic technique for precise colour alignment between layers, that was missing in Pixelmator, and I am afraid it is still missing in Pixelmator Pro.

@lahoda, @Gordon: I don't know if this meets all your requirements but...
You can mess about with individual channels of a layer by using the channel mixer to separate the the channels. In the image below I have:
Created two additional copies of the layer.
Set all the values in the Channel Mixer to zero apart from the colour I want to use.
Set the blend mode of the layers to Screen
Bumped red 10 px to the right and green 10 px to the left.
Will that do the job?
You can always recombine them using Merge

Cautionary Note
There's a bug in 1.0.8 that that will merge the layer effect (Channel Mixer) with the layer as soon as you paint on that layer. So... you would have to paint on that layer using only the colours of that layer or keep re-applying the Channel Mixer effect. (And you would immediately lose any information in the channels that are not being displayed ).